Using Flex-Algo for Segment Routing (SR) based Virtual Transport Network (VTN)
draft-zhu-lsr-isis-sr-vtn-flexalgo-06

LSR Working Group                                                 Y. Zhu
Internet-Draft                                             China Telecom
Intended status: Standards Track                                 J. Dong
Expires: 11 January 2024                                           Z. Hu
                                                     Huawei Technologies
                                                            10 July 2023

Using Flex-Algo for Segment Routing (SR) based Virtual Transport Network
                                 (VTN)
                 draft-zhu-lsr-isis-sr-vtn-flexalgo-06

Abstract

   Enhanced VPN (VPN+) aims to provide enhanced VPN services to support
   some existing or emerging application's needs of enhanced isolation
   and stringent performance requirements.  VPN+ requires integration
   between the overlay VPN connectivity and the characteristics provided
   by the underlay network.  A VTN is a virtual underlay network that is
   associated with a network topology, and is allocated with a set of
   dedicated or shared resources from the underlay physical network.  A
   VTN could be used as the underlay for one or a group of VPN+
   services.

   The topological constraints of a VTN can be defined using Flex-Algo,
   a mechanism to provide distributed constraint-path computation.  In
   some network scenarios, each VTN can be associated with a unique
   Flex-Algo, and the set of network resources allocated to different
   VTNs can be instantiated as layer-2 sub-interfaces or member links of
   the layer-3 interfaces.  This document describes the mechanisms to
   build the Segment Routing (SR) based VTNs using SR Flex-Algo and IGP
   L2 bundles with minor extensions.  This document updates RFC 8668 by
   defining a new flag in the Parent L3 Neighbor Descriptor in the L2
   Bundle Member Attributes TLV.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 11 January 2024.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Advertisement of SR VTN Topology Attributes . . . . . . . . .   4
   3.  Advertisement of SR VTN Resource Attributes . . . . . . . . .   5
   4.  Forwarding Plane Operations . . . . . . . . . . . . . . . . .   6
   5.  Scalability Considerations  . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Enhanced VPN (VPN+) is an enhancement to VPN services to support the
   needs of new applications, particularly including the applications
   that are associated with 5G services.  These applications require
   enhanced isolation and have more stringent performance requirements
   than that could be provided with conventional overlay VPN techniques.
   Thus these properties require integration between the underlay and
   the overlay networks.  [I-D.ietf-teas-enhanced-vpn] specifies the
   framework of enhanced VPN and describes the candidate component
   technologies in different network planes and network layers to

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   realize VPN+. VPN+ may be used to deliver IETF network slice
   services, and will also be of use in other generic scenarios.

   To meet the requirement of VPN+ services, a number of virtual
   transport networks (VTN) can be created, each is associated with a
   network topology, and is allocated with a set of dedicated or shared
   resources from the underlay physical network, so as to meet the
   requirements of one or a group of VPN+ services.  Another possible
   approach is to create a set of point-to-point paths, each with a set
   of network resource reserved along the path, such paths are called
   Virtual Transport Paths (VTPs).  Although using a set of dedicated
   VTPs can provide similar characteristics as VTN, it has some
   scalability issues due to the per-path state in the network.

   [I-D.ietf-spring-resource-aware-segments] introduces resource
   awareness to Segment Routing (SR) [RFC8402].  As described in
   [I-D.ietf-spring-sr-for-enhanced-vpn], the resource-aware segment
   identifiers (SIDs) can be used to build VTNs with the required
   network topology and network resource attributes to support VPN+
   services.  With a segment routing based data plane, SIDs can be used
   to represent both the topology and the set of network resources
   allocated by network nodes to a VTN.  For VTN-specific path
   compuation and instantiation, the SIDs of each VTN together with its
   associated topology and resource attributes need to be distributed in
   control plane.

   [I-D.dong-lsr-sr-enhanced-vpn] defines the IGP mechanisms and
   extensions to provide scalable SR based VTNs.  The mechanism in
   [I-D.dong-lsr-sr-enhanced-vpn] allows flexible combination of the
   topology and resource attributes to provide a relatively large number
   of VTNs with relatively small number of topologies.  In some network
   scenarios, the number of required VTNs may be small, thus a
   simplified solution to provide a small number of VTNs may also be
   desired.

   This document describes a mechanism to build the SR based VTNs using
   SR Flex-Algo [RFC9350] and IGP L2 bundle [RFC8668] with minor
   extensions.  With this mechanism, each VTN is associated with a
   unique Flex-Algo, and the set of network resources allocated to
   different VTNs are instantiated using layer-2 sub-interfaces or
   layer-2 member links of the L3 interfaces.  It can provide a
   relatively small number of VTNs, and can be considered as a
   transitional solution for the VTN deployment.

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   This document updates [RFC8668] by defining a new flag in the Parent
   L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV.
   [RFC8668] states that all bit fields not defined in that document
   "MUST be set to zero on transmission and ignored on receipt".
   Section 3 of this document defines a new flag and specifies when it
   should be set and how it should be processed.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Advertisement of SR VTN Topology Attributes

   [RFC9350] specifies the mechanism to provide distributed constraint-
   path computation, and the usage of SR-MPLS prefix-SIDs and SRv6
   locators for steering traffic along the constrained paths.

   The Flex-Algo Definition (FAD) is the combination of calculation-
   type, metric-type and the topological constraints used for path
   computation.  According to the network nodes' participation of a
   Flex-Algo, and the rules of including or excluding Admin Groups (i.e.
   colors) and Shared Risk Link Groups (SRLGs), the topology of a VTN
   can be described using the associated Flex-Algo.  If each VTN is
   associated with a unique Flex-Algo, the Flex-Algo identifier could be
   reused as the identifier of the VTN in the control plane.

   With the mechanisms defined in[RFC8667] [RFC9350], SR-MPLS prefix-SID
   advertisement can be associated with a specific topology and a
   specific algorithm, which can be a Flex-Algo.  This allows the nodes
   to use the prefix-SIDs to steer traffic along distributed computed
   constraint paths according to the associated Flex-Algo in a
   particular topology.

   [RFC9352] specifies the IS-IS extensions to support SRv6 data plane,
   in which the SRv6 locators advertisement is associated with a
   topology and a specific algorithm, which can be a Flex-Algo.  This
   allows the nodes to use the SRv6 locators to steer traffic along
   distributed computed constraint paths according to the associated
   Flex-Algo in a particular topology.  In addition, topology/algorithm
   specific SRv6 End SIDs and End.X SIDs can be used to enforce traffic
   over the Loop-Free Alternatives (LFA) computed backup paths.

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3.  Advertisement of SR VTN Resource Attributes

   Each VTN can be allocated with a set of dedicated or shared network
   resources on different network nodes and links.

   In order for a network controller or the ingress nodes to perform
   constraint based path computation for each VTN, the resource
   attributes of each VTN need to be advertised.  This way, the network
   controller or the ingress node can compute an SR-TE path in a VTN by
   taking both the Flex-Algo constraints and the resource attributes of
   the VTN into consideration.

   IS-IS L2 Bundle [RFC8668] was defined to advertise the link
   attributes of the layer-2 bundle member links.  In this section, it
   is extended to advertise the set of network resource attributes
   associated with different VTNs on a layer-3 link.

   The layer-3 link must have the capability of partitioning the link
   resources into different subsets and allocate them to different VTNs
   it participates in.  Each partition of the link resources can be
   instantiated as a layer-2 sub-interface, which can be seen as a
   virtual layer-2 member link of the layer-3 link.  If the layer-3 link
   itself is a layer-2 link bundle, the set of link resources allocated
   to a specific VTN may be provided by one or multiple physical layer-2
   member links.

   A new flag "E" (Exclusive) is defined in the flag field of the Parent
   L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV (25).

                0 1 2 3 4 5 6 7
               +-+-+-+-+-+-+-+-+
               |P|E|           |
               +-+-+-+-+-+-+-+-+

   E flag: When the E flag is set, it indicates each member link under
   the Parent L3 link is used exclusively for one VTN, and load sharing
   among the member links in different VTNs is not allowed.  When the E
   flag is clear, it indicates load balancing and sharing among the
   member links are allowed.

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   Note that legacy implementations of [RFC8668] will set the E flag to
   zero (clear) meaning that load balancing among component links is the
   default behavior.  Further, when a legacy implementation receives an
   E flag that is set, it will ignore the flag and so will assume that
   load balancing among component links is allowed even when the sender
   has requested it to not be used.  The Flex-Algo associated with the
   VTN can be defined that only nodes which support the E flag and the
   mechanisms defined in this document are included in the constraint-
   based path computation and packet forwarding of the VTN.

   For each virtual or physical layer-2 member link, the Admin Groups
   (AG) or Extended Admin Group (EAG) attribute MUST be advertised using
   the mechanisms as defined in [RFC8668].  This is for the correlation
   between the Flex-Algo specific forwarding entries and the layer-2
   member links of the VTN.  Other TE attributes as defined in [RFC5305]
   such as the Maximum Link Bandwidth attribute MAY also be advertised
   for the constraint-based path computation performed by the controller
   or the ingress nodes.  The SR-MPLS Adj-SIDs or SRv6 End.X SIDs
   associated with each of the virtual or physical layer-2 member links
   SHOULD be advertised according to [RFC8668] and
   [I-D.dong-lsr-l2bundle-srv6].

   In order to correlate the virtual or physical layer-2 member links
   with the Flex-Algo ID which is used to identify the VTN, each VTN is
   assigned with a unique Admin Group (AG) or Extended Admin Group
   (EAG), and the virtual or physical layer-2 member links associated
   with this VTN are configured with the AG or EAG assigned to the VTN.
   The AG or EAG of the parent layer-3 link SHOULD be set to the union
   of all the AGs or EAGs of its virtual or physical layer-2 member
   links.  In the definition of the Flex-Algo corresponding to the VTN,
   it MUST use the Include-Any Admin Group rule with only the AG or EAG
   assigned to the VTN as the link constraints, the Include-All Admin
   Goup rule or the Exclude Admin Group rule MUST NOT be used for a
   Flex-Algo associated with VTN.  This is to ensure that the layer-3
   link is included in the Flex-Algo constraint based path computation
   for each VTN it participates in.

4.  Forwarding Plane Operations

   For the SR-MPLS data plane, a prefix SID is associated with the paths
   calculated using the Flex-Algo corresponding to a VTN.  An outgoing
   layer-3 interface is determined for each path.  In addition, the
   prefix-SID also steers the traffic to use the virtual or physical
   layer-2 member link which is associated with the VTN on the outgoing
   layer-3 interface for packet forwarding.  A forwarding entry MUST be
   installed in the forwarding plane using the MPLS label that
   corresponds to the Prefix-SID associated with the Flex-algorithm
   corresponding to the VTN.  The Adj-SIDs associated with the virtual

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   or physical member links of a VTN MAY be used together with the
   prefix-SIDs of the same VTN to build SR-MPLS TE paths under the
   topological and resource constraints of the VTN.

   For the SRv6 data plane, an SRv6 Locator is a prefix which is
   associated with the paths calculated using the Flex-Algo
   corresponding to a VTN.  An outgoing Layer-3 interface is determined
   for each path.  In addition, the SRv6 Locator prefix also steers the
   traffic to use the virtual or physical layer-2 member link which is
   associated with the VTN on the outgoing layer-3 interface for packet
   forwarding.  A forwarding entry for the SRv6 Locator prefix MUST be
   installed in the forwarding plane for the Flex-algorithm
   corresponding to the VTN.The End.XU SIDs associated with the virtual
   or physical member links of a VTN MAY be used together with other
   types of SRv6 SIDs of the same VTN to build SRv6 paths under the
   topological and resource constraints of the VTN.

5.  Scalability Considerations

   The mechanism described in this document assumes that each VTN is
   associated with a unique Flex-Algo, so that the Flex-Algo IDs can be
   reused to identify the VTNs in the control plane.  Although this has
   the benefit of simplified control plane, it also has some
   limitations.  Firstly, it means that even if multiple VTNs share the
   same topological constraints, they still need to be identified using
   different Flex-Algo IDs in the control plane, then independent path
   computation needs to be executed for each VTN.  Secondly, the number
   of VTNs supported in a network may be dependent on the number of
   Flex-Algos supported, which is related to the number of Flex-Algos
   supported both in the protocol specification (which is 128) and the
   control plane overhead on network nodes under a spedific network
   scale.  Thus the mechanism described in this document is applicable
   to network scenarios where the number of required VTN is relatively
   small.  A detailed analysis about the VTN scalability and the
   possible optimizations for supporting a large number of VTNs can be
   found in [I-D.ietf-teas-nrp-scalability].

6.  Security Considerations

   This document introduces no additional security vulnerabilities to
   IS-IS.

   The mechanism proposed in this document is subject to the same
   vulnerabilities as any other protocol that relies on IGPs.

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7.  IANA Considerations

   This document does not request any IANA actions.

8.  Acknowledgments

   The authors would like to thank Zhenbin Li, Peter Psenak, Adrian
   Farrel and Gyan Mishra for the review and discussion of this
   document.

9.  References

9.1.  Normative References

   [I-D.dong-lsr-l2bundle-srv6]
              Dong, J. and Z. Hu, "Advertising SRv6 SIDs for Layer 2
              Bundle Member Links in IGP", Work in Progress, Internet-
              Draft, draft-dong-lsr-l2bundle-srv6-01, 24 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-dong-lsr-
              l2bundle-srv6-01>.

   [I-D.ietf-spring-resource-aware-segments]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Introducing Resource Awareness to SR
              Segments", Work in Progress, Internet-Draft, draft-ietf-
              spring-resource-aware-segments-07, 31 May 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              resource-aware-segments-07>.

   [I-D.ietf-spring-sr-for-enhanced-vpn]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Segment Routing based Virtual Transport
              Network (VTN) for Enhanced VPN", Work in Progress,
              Internet-Draft, draft-ietf-spring-sr-for-enhanced-vpn-05,
              31 May 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-spring-sr-for-enhanced-vpn-05>.

   [I-D.ietf-teas-enhanced-vpn]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Enhanced Virtual Private Network (VPN+)",
              Work in Progress, Internet-Draft, draft-ietf-teas-
              enhanced-vpn-13, 7 July 2023,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-
              ietf-teas-enhanced-vpn/>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [RFC8668]  Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri,
              M., and E. Aries, "Advertising Layer 2 Bundle Member Link
              Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
              December 2019, <https://www.rfc-editor.org/info/rfc8668>.

   [RFC9350]  Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
              and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
              DOI 10.17487/RFC9350, February 2023,
              <https://www.rfc-editor.org/info/rfc9350>.

   [RFC9352]  Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
              and Z. Hu, "IS-IS Extensions to Support Segment Routing
              over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
              February 2023, <https://www.rfc-editor.org/info/rfc9352>.

9.2.  Informative References

   [I-D.dong-lsr-sr-enhanced-vpn]
              Dong, J., Hu, Z., Li, Z., Tang, X., Pang, R., and S.
              Bryant, "IGP Extensions for Scalable Segment Routing based
              Enhanced VPN", Work in Progress, Internet-Draft, draft-
              dong-lsr-sr-enhanced-vpn-08, 11 July 2022,
              <https://datatracker.ietf.org/doc/html/draft-dong-lsr-sr-
              enhanced-vpn-08>.

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   [I-D.ietf-teas-nrp-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J.,
              Mishra, G. S., Qin, F., Saad, T., and V. P. Beeram,
              "Scalability Considerations for Network Resource
              Partition", Work in Progress, Internet-Draft, draft-ietf-
              teas-nrp-scalability-02, 2 June 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              nrp-scalability-02>.

Authors' Addresses

   Yongqing Zhu
   China Telecom
   Email: zhuyq8@chinatelecom.cn

   Jie Dong
   Huawei Technologies
   Email: jie.dong@huawei.com

   Zhibo Hu
   Huawei Technologies
   Email: huzhibo@huawei.com

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